1. Field of the Invention
Embodiments of the invention relate to control circuits and improved power factor power supply systems.
2. Related Art
A switching converter (switching power supply system) having an output of an AC (Alternating Current) power supply as an input makes switching devices connecting the input and the output carry out switching operations to thereby invert a full wave rectified AC input voltage to a DC output voltage at a desired level to supply the DC output voltage to a load. Such a switching converter is widely used in devices such as electronic devices.
In such a switching converter, for ensuring stabilization and safety of a commercial electric power system, a power factor improvement is required for switching converters with power consumption thereof exceeding 75 W, for example. By the power factor improvement, loads on facilities in distribution systems are reduced to permit reduction in electric power costs.
In an improved power factor switching converter, when a load is inductive, the phase of an input current delays to the phase of an input voltage. Thus, by carrying out an adjustment for making the input current become in phase with the input voltage, the power factor is made enhanced. The converter is also provided with an error amplifier comparing an output voltage with a reference voltage and a phase compensation circuit for compensating the phase of an error signal as a result of the comparison in the error amplifier.
For obtaining a high power factor, the value of the gain of the error amplifier and the value of the phase compensation constant (time constant) of the phase compensation circuit are to be established to adequate values. This, however, caused a slow transient response when an input voltage became particularly low in a circuit with the phase compensation circuit being additionally provided to sometimes lower the response of the circuit.
For improving the delay in the transient response, a technique is proposed with which an output voltage is detected and, when the overshoot or undershoot in the detected voltage accompanied with the transient response becomes large, the gain of an error amplifier is made promptly increased. See Japanese Patent Application No. P-A-2011-211828 (also referred to herein as “Patent Document 1”).
With such technique as is proposed in Japanese Patent Application No. JP-A-2011-211828, however, when the input voltage is high, the power supply gain also becomes high. Hence, there was a problem in that the further prompt increase in the gain in the error amplifier with the input voltage being high might cause the oscillation in the output voltage. Moreover, when the input voltage is low, there was also a problem with respect to the undershoot of the output voltage in that an adequate value of a phase compensation constant was difficult to be established to make the circuit design difficult.
Thus, as described above, certain shortcomings exist in the related art.